The invention relates to a transfer printing ink for marking on an impervious workpiece and particularly, although not exclusively, to a transfer ink for printing a coded marking on a glass envelope portion of a cathode-ray tube (CRT).
U.S. Pat. No. 4,327,283 to P. M. Heyman et al., issued on Apr. 27, 1982, discloses a coded machine-readable marking formed by abrading a bar pattern through a top coating formed on an undercoating affixed to an external surface of a glass faceplate panel of a CRT. The method requires forming a black undercoating and then depositing a white layer on the undercoating. A bar code pattern is formed by selectively removing portions of the white coating to form adjacent areas having different reflecting characteristics. The coatings are then fired to fuse the coatings to the surface and to increase their chemical durability. The bar code pattern may be used for item identification and/or for the control of subsequently performed processes. The coatings, which are either frit-type or alkali silicate-type, are applied by spraying or screening and the white coating is removed by an abrasive stream or by a laser. Such a label is labor intensive to produce and therefore costly.
Glass printing inks, such as the ink disclosed in U.S. Pat. No. 3,096,184 to J. L. Gallup, which are applied by a stamping process are most cost effective than the coded marking of the Heyman et al patent, but such inks are unsuitable for use on CRT's because the ink must be fired at 550.degree. C. to drive off the vehicle of the ink mixture.
Printing of bar code labels using transfer inks and a pad printer of the type described in U.S. Pat. No. 4,060,031 to Philips on Nov. 29, 1977 offers considerable advantage over prior printing methods since the pad is made of an ink repellent surface which substantially completely transfers the ink to the printed material. However, prior transfer inks also are incompatible with CRT manufacturing because of improper viscosity, poor pad-transfer characteristics, incompatibility with the relatively low cathode-ray tube processing temperatures, poor bond strength, long drying times or poor reflectivity.